This invention relates generally to inspection of nuclear reactors, and more particularly to measuring an electrochemical corrosion potential (ECP) of a material sample.
The reactor pressure vessel (RPV) of a boiling water reactor (BWR) has a generally cylindrical shape and is closed at both ends, for example, by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
The radiation field in an operating boiling water reactor (BWR) generates strong oxidizing species, such as oxygen and hydrogen peroxide. The presence of such oxidizing species contribute to the intergranular stress corrosion cracking (IGSCC) of 304 stainless steel components within the reactor. IGSCC is known to be a major environment-related material performance problem within BWRs. Lowering concentrations of ionic impurities and oxidizing species in the reactor water may facilitate mitigation of IGSCC. The electrochemical corrosion potential (ECP) of stainless steels and other active metals is known to be controlled by dissolved oxygen, hydrogen, and hydrogen peroxide concentrations in the BWR water and the hydrodynamic flow conditions within the water path. In order to evaluate or predict materials performance (including IGSCC as a function of time), it is desirable to know the ECP value of the structural materials that are exposed to high temperature water within the reactor pressure vessel.
In moderate hydrogen water chemistry (HWC), hydrogen is added to the feed water of a BWR to facilitate mitigating IGSCC. A purpose of adding hydrogen is to reduce the concentrations of dissolved oxidants in the reactor water and thus lower the ECP to a value that is less than approximately −230 mV, measured against a standard hydrogen electrode (SHE), at which IGSCC susceptibility is reduced. Hydrogen levels in the reactor water are maintained in excess of the stoichiometric amount needed to react with either oxygen or hydrogen to form water. However, there are several side effects of the moderate HWC application, such as increased nitrogen-16 carry-over to the turbine and higher cobalt-60 deposition rates. Also, the critical ECP value that is needed to prevent IGSCC is difficult to achieve in highly oxidizing and/or high water flow regimes.
Noble metal chemical addition (NMCA) improves the catalytic properties of metal surfaces for the recombination of either hydrogen/oxygen or hydrogen peroxide/hydrogen to form water, NMCA allows low metal surface ECP values to be achieved at lower hydrogen addition rates. This catalysis reduces the oxygen concentration at the metal surface to approximately zero, thus causing the ECP to drop to its thermodynamic minimum of approximately −550 mV SHE in pure water at 288 degrees Celsius. To achieve a stoichiometric excess of hydrogen, feed water hydrogen addition is adjusted to achieve a hydrogen:oxygen molar ratio of greater than 2:1, or a hydrogen:oxygen weight ratio of greater than 1:8 in the reactor water. Injection of NMCA chemicals, directly into the reactor water deposits noble metals onto the surfaces of reactor components that are exposed to the reactor water. The surfaces of the reactor components are typically covered with an oxide outer layer. The noble metals are deposited onto the oxide layer, thus providing a catalytic site for both the hydrogen/oxygen and hydrogen/peroxide recombination reactions. The ECP value needed to ensure protection of components from IGSCC can then be achieved through the addition of smaller amounts of hydrogen, thus avoiding many of the negative side effects that are frequently encountered at higher hydrogen concentrations. Evaluating the effectiveness of noble metal deposited within existing cracks is beyond current technology.